1. Field of the Invention
The present invention relates to a centrifugal blower provided with a sub blade, which is suitable for use in an air conditioning apparatus for a vehicle.
2. Description of Related Art
A centrifugal blower is constructed by a centrifugal multi blade fan which includes a plurality of blades disposed around a rotating axis, a scroll casing defining an air inlet and an outlet, and a drive motor for rotating the fan. A blower performance such as an air discharging ability and a noise reducing ability depends on its fan shape and casing shape, thus it is necessary to consider these factors so as to improve a blower performance.
Japanese Unexamined Patent Publications No. 5-306699 and No. 5-302600 disclose a centrifugal blower in which an opening area of an air intake opening is enlarged for increasing an air flow amount. In this centrifugal blower, as shown in FIG. 15, outer periphery of the air intake opening 76 is arranged at the outside of the inner edge 72a of the blade 71.
In this blower, because the air inlet is enlarged, adding to a main air which is introduced from the inner edge 72a of the blade 71, a secondary air which is introduced from the upper end 71a of the blade 71. Then, both the main air and the secondary air are discharged from the entire outer edge 72b of the blade 71, thereby increasing the air discharge amount.
However, a path through which the secondary air flows is shorter than that of the main air. Thus, a kinetic energy imparted to the secondary air is small.
Accordingly, when the blower is used in an air passage system with a low pressure loss, the air flow amount can be increased as described above. However, when it is used in an air passage system with a high pressure loss, the air flow amount cannot be increased.
Here, FIG. 16 shows a result of a simulation of the air which flows between the blades, and a length of a vector (arrow) denotes an air flow velocity at a starting point of the vector.
That is, in the air passage system with a high pressure loss, as shown in FIG. 16, a reverse air which flows from the outer edge 72b to the inner edge 72a at a location where the secondary air flows, i.e., a dead air flow area (eddy flow), arises.
Thus, a noise caused by a friction between the reverse air flow and the main air flow arises. Further, the main air flow amount and the secondary air flow amount are reduced by the friction with the reverse air flow, thereby reducing the total air flow generated by the blower.